Various researches have been conventionally performed on methods for managing encrypted contents. For example, a technique for assigning a random number that is set for a management ID and a file name before encryption to a hash function and using the obtained hash value as the management ID has been known as disclosed in, for example, Japanese Laid-open Patent Publication No. 2006-285697 (pages 1 to 5 and 9, FIGS. 1 and 11).
Further specifically, a file management system disclosed in Japanese Laid-open Patent Publication No. 2006-285697 includes a file storage device that stores an encrypted file, a file name before encryption, a random number for management ID, and the type of hash function and a key management device that manages a decryption key for decrypting the encrypted file and a management ID in association with each other. Under such a configuration, the file storage device transmits the file name before encryption, the random number for management ID, and the type of hash function to the key management device. Then, the key management device computes a hash value by using the received type of hash function from the received file name before encryption and random number for management ID. When there is a management ID identical to the computed hash value, the key management device transmits the corresponding decryption key to the file storage device.
Moreover, a technique for backing up data within a mobile device in a backup unit connected via a network has been known as disclosed in, for example, Japanese Laid-open Patent Publication No. 2006-211051 (pages 1, 2, and 4, FIG. 1).
However, the conventional art has a problem in that the extent of security of an archive system is not sufficient.
For example, in Japanese Laid-open Patent Publication No. 2006-285697 described above, because a hash value is computed by using a random number for management ID and a content name and is utilized as a management ID, the random number for management ID and content name leak and thus a decryption key leaks. As a result, the extent of security of an archive system is not sufficient. Moreover, in Japanese Laid-open Patent Publication No. 2006-285697, because what is used is a hash value computed from a content name and the like, interpolation cannot be detected when contents are directly interpolated with unjust data. Therefore, the extent of security of an archive system is not sufficient.